CONSTITUTION OF SALTS. Jfij 



It has become necessary to recognise three classes of oxygen- 

 acid salts, which in the language of the old theory contain, one, 

 two and three atoms of base to one of acid. 



1. Monobasic salts. The, great proportion of acids, such as 

 sulphuric, nitric, &c. neutralize but one atom of base, and form, 

 therefore, monobasic salts. But this is not inconsistent with 

 an acid's forming two series of salts with the same base or class 

 of isomorphous bases. Thus there appear to be two well 

 marked classes of sulphates of the magnesian oxides, which 

 agree in having one atom of base, but differ essentially in the 

 proportions of combined water which they affect. In one series the 

 sulphate is combined with one, three, five or seven atoms of 

 water. Copperas (a sulphate of iron), epsom salt (a sulphate 

 of magnesia), blue vitriol (a sulphate of copper), and most of 

 the well known magnesian sulphates belong to this class, which 

 may be called the copperas class of sulphates. All the mem- 

 bers of it are very soluble in water, and form double salts with 

 sulphate of potash. The other series affect two, four and six 

 atoms of water. They are less known, but appear to be of 

 sparing solubility, and to be incapable of forming double salts 

 with sulphate of potash. Gypsum or sulphate of lime belongs 

 to this class, which may, therefore, be called the gypsum class 

 of magnesian sulphates. Sulphate of iron crystallizes from 

 solution in sulphuric acid with two atoms of water, with the 

 crystalline form and sparing solubility of gypsum. Dr. Kane 

 obtained a sulphate of copper with four atoms of water, by 

 exposing the anhydrous salt to the vapour of hydrochloric 

 acid, which appears to be the second term in this series ; and 

 Mitscherlich still maintains the existence of a peculiar sulphate 

 of magnesia containing six atoms of water of crystallization, 

 which will constitute the third term. It is evident that the 

 cause of such double classes of salts is as deeply seated as 

 that of dimorphism, and hence, possibly, the magnesian sulphate 

 itself, which exists in the two classes, is not the same in its 

 constitution with reference to heat. 



2. Bibasic salts. That class of phosphates, which received the 

 name of pyrophosphates, was the only class of salts, known till 

 lately, in which one atom of acid neutralizes two atoms of base. 

 But according to the recent results of Fremy which have been 

 favourably reported upon by Dumas,* the classes of tartrutes 



* L'lustitut, May 1838, p. 141. 



